Samarium compensator method for nuclear reactor fuel



United .States Pnfo Y 'f l2,843,539 *Y u e,

SAMARIUM coMPENsAToR MErHoD non l Y NUCLEAR REAcroRrUEL Ira Bornstein, Balboa, Califaassignor to North American Aviation, Inc.V

No ufawig. Applicafiim May 16,1955 f f Yserial No.s0s,790

e claims. (c1. rtl4-1542) 'This invention relates t nuclear reactors, and part'icula'rly 'to a fuel'mixture Vfora nuclear reactor. o

If a nuclear reactor isto operate'in a Vsteady state,

" thef rate'ofloss of neutronslbyjescapeVandfby capture. The 'critical size is not a constant'for yal1 Vreactors but depends lonv the ,isotopic composition of the'nuraniuiruA theproportion of moderator and the presence of various circumstances causing parasitic capture of neutrons.

, the atoms formed are-not eifcctively rmoved frode Y `the reactors core by radioactive decay kwltrhmth'Arm???,

Y the effective multiplication factor must remain at unity, 'e

" negligibly low absorption crossesection for thermal neu-' 1` r ...feared Jewels?.

runningoftime. t .Y is- Some of the Vnuclides formedvras ssion products haveV comparatively hightmicroscopic absorption cross-sections, t a'a, for thermal neutrons.V The microscopic absorptionv vcross-sectionfor thermalvneutrons' savmeasure of the,"

ability'of Va particular materialto'absorb incident thermal 1 neutrons: Specifically, it is theprobability of the occur-,

Vrence of this absorption4 per atomV per' incidentthermal neutron per square centimeter. `'The barn-is` commonly '-1 usedas the unit of cross-section. Samariun1-149- has a' cross-section, tra', 'ofY about'5.3 104 barns.` A crossf section of AthisV magnitude islarge enough. to 'cause aff serious disturbance 'of the neutron Veconomy Vof the 11el actors Thus, the samarium-ilt? inthe reactor acts asa poison, Yremoving aj certain kpercentage oftheneutrons from the chain reaction thereby necessitating an excess".V amount of iissionable, material to compensate. for

loss -of neutrons. Upon absorption of a neutron; Samar-'V Y lum-149 becomes sainariun1-150 which invturn'has a',

trons; Since samarium-l49is continuously being formedT in the reactor core' by the ssion process andalso is vconi-,f tinuously being removed by absorption of. neutrons, a

conditionfo'r` equilibrium isl eventually reachedvjin anyv particularreactor invwhich the rate ',of `formation exactly;Y

VThe neutrons produced' by fission in av nuclear reactor arelost or ,usedin: one of ,the @following four classes result,of theV (it, y).y reaction vvithuranium 238; third,

fission capture of V slow neutrons. yby',uranium-,235 iand of j A fast'yneutrons by both uranium-238 and uranium-235; and fourth,` nonission capture, often referred `,toas-parasiticcapture, byfvarious materials vrWithin the neutron,

A flux, in cludingfthe moderatonstructural material, coolant,various extraneous `substances v such as impurities in l'the uranium-and in the moderator, the control units and'ssion products. Itistheilatter nonssion capture of. neutrons. bythe fission products, vparticularly sarnarium149, which is considered below. For ease of identiiication ofthe various yelements and isotopes of a particular. element, the atomsof a particular nuclide or isotope' are identified by the elements name yfollowed by thel mass number. 'oIhus,-1'1ranium'`235 describes the atomsof the uranium element having a mass number vlof l2.135, and sama'riuml149f describes the, atoms Yof the samarium elementhaving va mass number/of 149,

quantity'ofssionable materialfin the reactor." f. Y o fhreactions'; First, complete loss` of:gneutronslby escap-V e .Y ing fromi the system; second, vnoniissionl captureasgja equals fthe rateY ofjremovalof sa1`narium-149.V VAs ,si shown *be1ow,-the` equilibrium steady'state, samariurnh 149 concentration in` agreactorxis.independentjof the neutronfluxwand is dependent solely on the.V type' a di If Sl is the' concentration of samari1. r1mrlt4`9 *nucleY :per: cm.3 at any instantgthe rate of removalfo'f samariuim' 149by neutron :capture formingA samariumlSOgis ja' p S, wlilere'aa is ythe vmicroscopic absorption"crosssection'in cm2 of samarium-"l49-`for lthermal neutron vcapture and g5 i'sV the thermal iuxk ofthereactor in'neutrons'pr cm.2

per second. The rate of f orrnationofthe samariunzi-lll-9V if 2f'- is the macroscopic'l (thermal neutron) vcross-fsection` Y second. Therefore, the rate of change ofsamariumf14-9 1 If thereactor has beenloperatinglfor some-time,` theU l Yis,Well'-kxaovfn, the fission proeess, in addition to produclngfneutrons, also -produces numerous different iission products, i 'lflies'e V4fission'v products include theYA fission fragments produced directly vby fission asv well` asv the numerous ditferentdecay products of thoseviissionfrag-Y slzero. .'Indicatirig 'thisequilibrium'.concentratioxfas `Sc, Y kand the corresponding (steady state] ux byY en, Equation mentes?. 'Ihere is af fixed,Y lprobability l`that the ssion I Vfof 'a""'p'articular ssionable material Vwill form either di-A V4active decayfofjneodymium-149 and promethium-'l'49 inaccordancevvithjthe decay equation:

' uvhere'the lives for ,radioactive decay of neodymiuinconcentration is: Y 4 f Y l l! I ,hy

amie-eef concentration. of :samarium-,149 has attained an equlibriumyaluefand r- A 1l U vdS@Y ltreduces to:

Vmariur'n-149 is independentbf the neutronl ux in the gre'- iteratively-.f-.;iea maritim-149 isa'astable,Y nuciide, 1f

actor. However, the time required for a particular re-v actor to reach equilibrium does depend on the magnitude of the neutronflux, Yqtybeing-approximatelye'qual to S/aab; seconds. Thus,l itl takes about' ll days for ,the equilibrium condition to exist'- for a nuclear reactor operating with a linx as high as 1014 neutrons per cm.2 per second. If the flux is 1013 neutrons per cm.z per second, it takes about sixty days to reach the equilibrium condition. Lower values `of average ux result in longer periodsy of time before a, state of equilibrium is attained.

In order to compensate for the excess reactiv-ity `of the core` of a nuclear, reactor during the-period of time required for the above-mentioned equilibrium concentration of samarium-149 to be attained, past reactors have. been designed with an excess amount of control elements to thereby permit initial insertion of a neutron poison in, an amount equal, in effect, to the disturbance of the neutron economy subsequently caused by the samarium-V 149 poison inthe core. This compensation for the initial lack of samarium-149 atoms in the core of the reactor was, accomplishedv by an initial deeper insertion of the control elements of the reactor with a corresponding gradual withdrawal of the control element to compensate for subsequent samarium-149 build-up. It is to be noted, however, that an initial amountof excess reactivity equal toA about 1% must be built into the reactor core to compensate for the steady state samarium poisoning. This additional amount of excess reactivity is a possible source of danger in the operation of the nuclear reactor, particularly during the samarium build-up time. Thus, if by accident, carelessness or design, the reactor control rod is removed from the vicinity of the core during this samarium build-uptime, the reactor is subjected to a much more violent runaway than occurs if the equilibrium condition has been attained. Thus, after the initial starting of the reactor, there is, in addition to the normal amount of excess reactivity built into the reactor for purposes of controlling the power level, an additional 1% excess reactivity built into the reactor for samarium compensation. This latter-excess reactivity may well be sulcient to cause a runaway of su'icient magnitude to render the reactor inoperative. e

`It is -thereforean object of this invention to provide a fuel mixture for a nuclear reactor which removes from the control of the reactor operator the excess reactivity required to compensate for steady state fission product poisoning.

' It is another object of'this invention to provide a fuel mixture for a nuclear reactor in which there is initially inserted in the nuclear fuel the steady state samarium- A149 concentration. Y It is another object of this invention to provide a method for initially compensating for the steady' state samarium poisoning of a nuclear reactor.

, It is a further object of'this invention to provide a fuel mixture for a nuclear reactor comprising a uniform mixture of about one atom of natural samarium to every 950 atoms of uranium-235 in said fuel.

It is another object of this invention to provide a fuel mixture for a nuclear reactor comprising a mixture of about .068% by weight of SmzOa in'U2`35O2. f

Other objects of this invention will Vbecome apparent from the following description.

It is to be noted that the over-all safety of operation of a nuclear reactor can be increased by reducing its available excess reactivity., Thus, it is desirable to remove as much as'possible of the excess reactivity from the control of the operator and, if possible, from reliance on any mechanical or electrical instrumentation. The samarium compensator of this invention accomplishes this reduction in the available excess reactivity during the time required for samarium built up by initially inserting into the fuel elements of the nuclear reactor an amount of samarium-149 which is substantially equivalentto the-amount of steady state samarium poisoning produced by the ssion process.

As previously pointed out, theequilibrium steady state samarium concentration in a nuclear reactor is independent ofthe neutron iiux and is. dependent only on the quantity of ssionable material in the reactor. lt is further noted that the lack of suicient samarium-149 in the fuel is only present when the reactor is initially placed in operation or when a new fuel element is inserted in the reactor core. The ratio of the number of atoms of samarium-149 to the number of atoms of uranium-235 in the fuel elements ofa nuclear reactor when the samarium concentration is in the steady state equilibrium is found by the following formula:

neutron' economy inthe fuel, is:

Sm t 1%950 (4) Although metallic samarium has been isolated, sarnarium oxide (Sm2O3) is more readily available in very pure form. Most fuel elements presently used in nuclear reactors utilize uranium dioxide (U02) as a source of uranium. The uranium in this oxide is of varying degrees of enrichment in the uranium-235 nuclide. The ratio by weight of natural samarium oxide to uranium- 235 dioxide (U235O)2, which is equivalent to Equation 3 in its effect on the neutron economy in the fuel, is:

Uranium-235 dioxide 4includes those molecules of uranium dioxide in which the uranium atom is the uranium- 235 isotope. Fuel elements containing about .068% by weight of Sm'zOgveryA uniformly disbursed in U02 can be fabricated by standard powder metallurgy procedures well-known to those skilled in the art. Thus, powdered samarium oxide may be physically mixed in powdered uranous oxide in the above-mentioned proportions, mixed with a suitable binder, such as powdered aluminum, and compacted into a fuel element of the desired shape. Such fuel elements are usually further provided with an aluminum cladding Vto protect the fuel from the coolant.

Therefore, the initial mixture of one Aatom of samarium-149 to every 6880 atoms of uranium-235, either in the fuel of the nuclear reactor before its initial start-up or in a Yfuel element which is to be` inserted into an operating or temporarily shutdown reactor, effectively eliminateshthe excess reactivity in the reactor during the samarium build-up time. Either pure .natural samarium or a samarium compound maybe used to provide the desiredI ratio of Asamarium-149 to uranium-235. It is to be noted/that the ratios given are optimum values and are not intendedby Way of limitation. A greater or lesser ratio of samarium-V149 to uranium merely goes to the degree of compensation and not to the operativeness ofthe mixture. Y e 4. ,In order to decrease. the excess reactivity available in a thennalnuclear reactor the Vsamarium-.149 artificially added by mixing natural samarium in the `issionable material in'thefuelfelement should be less than twice the steady state samarium concentration in the reactor. It isfto be noted that if Ythe.samarium-149 concentration articiallyfadded to the fuel element is avalue 'less than a steady state Yc'olicentration there is a proportionate amount `of excess" reactivity available in the nuclear reactor during the samarium-149 build-up time. I f, however, the samarium-149 concentration artiicially added to the fuel element is more than the steady state concentration there is a proportionate amount of excess reactivity available in the reactor after the samarium-149 build-up time. This latter feature is apparent since the samarium-149 artificially added is eventually converted to samarium-150, leaving only the abovementioned steady'state samarium-149 production and removal. Thus, in the above example in which the optimum ratio of natural samarium to uranium-235 is 1 atom tol 950 atoms, there is a reduction in the excess reactivity of someidegree if this ratio is any value less than 2 atoms to 950 atoms.

In the above discussion, the uranium-235 isotope Vof the i uranium element is used by way of example. However,

j other nuclides which are iissionable by thermal neutrons may be utilizedY as ssionable materialin the fuel elements. Thus, the uranium-233 isotope of the uranium element and Ytheplutonium-239 isotope of the plutonium yelement also fission when subjected to thermal neutrons and further produce as a fission product samarium-149 although in dierent fractions' of yield than that of vuranium-235 The ratios of the number of atoms of samarium-149 tothe lnumber of atoms of uranium-233 (Um) or plutonium-239 (Pu`239) in the fuel elements of 'a thermal nuclear' reactor when the samarium concentration is in the steady state of equilibrium is found by the following formula: Y

Smm 'Y U 'ma-:1455), Y .(62

vandV Since only 13.8% of the atoms of natural samarium are the samarium-149 nuclide, it is necessary to initially add to the core of the reactor a mass of Vnatural samarium` equivalent to about seven times the mass of the steady state samarium-149 component of thereactor. Y It should be noted that the samarium-149 thus articially added to the nuclear fuel is gradually lost and replaced by ission Y product samarium-149 by neutron absorption and conv 3Q in which the 'y2 and '73 are the fractions of yield of y samarium-149 as a ssion product from the fission of Y uranium-233 and plutonium-239, respectively, and ,ini -which q2 .and an, are the microscopic ssion (thermall neutron) cross-sections rversion to samarium-150 in the manner previously described. j

Although this invention has been described and illustrated in detail, it is to be clearly understoodthat the same is by way of illustration and example only and is not to be taken by way of limitatior'uthe spirit and scope of this invention being limited only bythe terms of the appended claims.

I claim:

1. The method of compensating for the excess reactivity provided in a thermal nuclear reactor duringthe period of samarium-149 build-up comprising initially distributing in a predeterminedv pattern a mass of naturalV samarium to provide in the core of said reactor a Vmass of samarium-149 substantially equal to the mass ofr steady state samarium-149 in said reactor.

2. The method of compensating for the excess reactiv-l ity provided in a nuclear reactor during the period of samarium-149 build-up comprising initially distributing in a predetermined pattern a mass of natural samarium inthe core of said reactor equivalent to about seven times the mass of the steady state samarium-149 component in said reactor.

3. In a nuclear'reactor utilizing uranium-235 dioxide as -a fuel, themethod of Vcompensating for the excess reactivity provided in said nuclear reactor during the` period ofY samarium-149 build-up comprising initially uniformly distributing samarium oxide in the# fuel elements of said reactor in the ratio of about .068% by y' weight of samarium oxide to uranium-235 dioxide.

reactivity of the reactor is less than twice the-optimum 45 value given above, that is, the atom ratio of natural samarium to the iissionable material is less than 32(100/135) or about 14.5 times the fractional yield,

'1, of samarium-149 from the fission of the iissionable ma- References Cited in the'ile of this patent UNITED STATES lPATEN'I'S 2,708,656 Fermi et al. May 17, 1955 FOREIGN PATENTS 648,293 Great Britain Jan. s, 1951 OTHER REFERENCES The 'Elements of Nuclear VReactor Theory, by Samuel 1terial multiplied'by theratio of the microscopic issionV 50 Glasstone andMilton C. Edlund, D. Van Nostrand Co.,

R (14.5)1i (s) N. Y., 1953, PP. 329-339. Y

Introduction to Nuclear Engineering, Richard Stephenson, McGraw-Hill Book Co., N. Y., 1954, pp. 270-274. 

1. THE METHOD OF COMPENSATING FOR THE EXCESS REACTIVITY PROVIDED IN A THERMAL NUCLEAR REACTOR DURING THE PERIOD OF SAMARIUM-149 BUILD-UP COMPRISING INITIALLY DISTRIBUTING IN A PREDETERMINED PATTERN A MASS OF NATURAL SAMARIUM TO PROVIDE IN THE CORE OF SAID REACTOR A MASS OF SAMARIUM-149 SUBSTANTIALLY EQUAL TO THE MASS OF STEADY STATE SAMARIUM-149 IN SAID REACTOR. 